Key ModSummary cache by UnitId as well as FilePath

Multiple units can refer to the same files without any problem. Just
another assumption which needs to be updated when we may have multiple
home units.

However, there is the invariant that within each unit each file only
maps to one module, so as long as we also key the cache by UnitId then
we are all good.

This led to some confusing behaviour in GHCi when reloading,
multipleHomeUnits_shared distils the essence of what can go wrong.

Fixes #22679